Unlike flowering plants which grow from seeds, ferns grow from tiny, dust-like spores. The spores come from specialized structures on the undersides of the leaves. You can propagate sword ferns (Polystichum munitum) from spores or by dividing a large plant. Sword ferns are substantial, evergreen plants 3 to 6 feet tall and wide, native to forests of the Northwest United States. They're hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8.
Sword fern grows new fronds each year from the crown of the plant, which is where the roots and fronds come together near the top of the soil. The new fronds uncurl from a tight coil, resembling a fiddle neck as they grow. Older ferns produce offsets, forming a clump.
When sword ferns produce spores for the first time, they're between 1 and 5 years old. To collect spores, look at leaf undersides in summer. Raised circular structures called sori form rows to each side of leaflet midveins. When the spores are ripe, structures in the sori break open to release spores.
Hold a paper bag under a fern frond that's releasing spores and shake the frond to deposit spores into the bag. If you're not going to germinate them right away, store them in glassine envelopes placed in an airtight, moisture-proof container at 33 to 40 degrees Fahrenheit.
Spores germinate as soon as they are ripe, so you can sow them right after collecting them in summer. You can also sow stored seed the next spring. Prepare a 6-inch pot that has drainage holes. Wash it in hot, soapy water, rinse it in clear water, and then soak it for at least five minutes in a mixture of 1 part household bleach to 9 parts water. Rinse the pot thoroughly and let it dry. Fill the pot within 1 inch of the rim with moistened new peat moss.
Scatter the spores evenly on top. Don't cover them with potting medium. To germinate, spores must be kept moist and the humidity high, so put a clear plastic bag over the pot. Place the pot in bright, indirect light at temperatures between 59 and 86 degrees F. Keep the potting medium moist but not wet. Wipe the condensate from the inside of the bag occasionally.
In a few weeks, the small, flat, heart-shaped gametophyte develops from the spore. It bears eggs and sperm, which unite to form the sporophyte, or spore-bearing fern, which is the stage you plant in the garden.
Occasionally mist the gametophytes with water, keeping the plastic bag on the pot. After a few weeks, the sporophyte forms, with little fern fronds uncurling from the gametophyte. The gametophyte eventually withers away as the new fern grows. Gradually open and then remove the plastic bag as the fern grows.
Young Sword Fern Care
As new ferns in the seed pot become large enough to handle, usually a few weeks after the fronds form, prick each one out of the growing medium. Use a clean slender tool such as a popsicle stick to transfer the ferns, taking extra peat moss along with the roots. Try not to disturb each root mass.
Put each fern into a 3-inch pot with drainage holes, with peat moss as the growing medium. Water thoroughly. Keep the fern in bright light and water it regularly to keep the soil moist but not soggy. Transplant to larger pot sizes as the fern grows. It takes about 2 years for the fern to be ready to plant in the garden.
When a sword fern is large and somewhat crowded, divide the clump. Dig the fern out of the ground or remove it from the pot in spring as new growth is showing. Shake off excess soil. Use a sharp knife wiped clean with a cloth soaked in rubbing alcohol to prevent disease spread. Cut the clump into pieces, each piece with at least one clump of leaves that has several fronds and buds.
Replant the divided pieces back into the garden or into separate pots that have drainage holes, keeping them in a shaded area. Water the transplants thoroughly, and mist them occasionally in addition to watering them regularly for the next few weeks.